Western Australia will reach 5 million by 2066, but most will live in Perth.
Estimates released this morning by the Australian Bureau of Statistics show that the state's population growth will drop sharply in the next few years, while rural areas will see a decline in population.
Western Australia now has a population of 2.6 million, and by 2027 it will be 2.9 million.
After that, depending on the birth rate, the rate of emigration abroad and the rate of emigration from other states, the population of Western Australia will reach 3.6 million in 2042 and 4.9 million in 2066.
The forecast is much lower than 2013, when the mining boom was the fastest-growing state in the country.
Perth currently accounts for 79% of Western Australia's population. Perth has overtaken Canberra as the country's most populous capital.
The Bureau of Statistics reckons Perth will make up 81% of the state's population in 2027.
This is because the population growth in rural areas of Western Australia is slow.
Immigration is important to Western Australia's population growth, but the Bureau of Statistics expects the birth rate in Western Australia to continue to outpace mortality by 2066.
The population of Western Australia will grow, and it will grow old.
The middle age in Western Australia is now 36.6 and will be in its 40s by 2066. The population over 85 will double from today to 3.6% of the population, while the proportion of children will fall from 20% to 16%.